The present invention relates to a particle-measuring system that is mounted on a processing unit for forming a film on a semiconductor wafer by using a gas, and that measures the number of particles included in an exhaust gas discharged from the processing unit.
Generally, in the manufacturing of semiconductor integrated circuits, various kinds of processing units are used for processing semiconductor wafers (hereinafter to be referred to as wafers) as objects to be processed at various manufacturing stages, including a film deposition (CVD: chemical vapor deposition) process, thermal oxidation and impurity diffusion processes, an etching process, a film forming (sputtering) process, a thermal processing process, etc.
In the film forming process, thin films such as a silicon oxide (SiO2) film, a silicon nitride (SiN) film, and the like are deposited as insulation layers or insulation films on the surface of the wafer using, for example, a CVD unit. For forming wiring patterns and embedding trenches, thin films of tungsten (W), tungsten silicide (WSi), titanium (Ti), titanium nitride (TiN), titanium silicide (TiSi), etc. are deposited.
When these processing systems are used to carry out each processing, it is necessary to avoid as far as possible the generation of particles that become the cause of reduction in product yield.
Therefore, a particle-measuring system is installed on the processing system in order to real-time monitor the state of generation of particles within a processing chamber or in order to know the timing for cleaning the processing chamber. Particularly, in the film-forming system such as a CVD system or a sputtering system, there occurs an adhesion of unnecessary films onto the inner wall of the processing chamber or onto the surface of the parts. These unnecessary films are disposed and accumulated within the chamber during the film-forming process. These unnecessary films are easily peeled off at the next film-forming cycle, and particles are easily generated. Therefore, it has been important to monitor the volume of particles generated during the film-forming process.
One example of a processing system having a conventional particle-measuring system will be explained with reference to FIG. 18.
A mounting table 4 for mounting a wafer W is provided inside a processing chamber 2 of almost a cylindrical shape, and a transmission window 6 made of quartz glass is disposed on the bottom of the chamber. A plurality of heating lamps 10 are disposed on a rotary table 8 below the transmission window 6. Heating beams irradiated from these heating lamps 10 are transmitted through the transmission window 6 to heat the wafer W on the mounting table 4.
A shower head 12 for introducing a processing gas such as a film-forming gas into the processing chamber 2 is provided on a chamber ceiling that faces the mounting table 4. Four exhaust openings 14 (only two openings are shown in the drawing) disposed with approximately equal intervals are provided on the periphery of the bottom of the processing chamber 2. Each of these exhaust openings 14 is connected to an exhaust pipe 16 extending downward.
Respective discharge sides of the exhaust pipes 16 are assembled into one, which is then connected to one absorption side of an assembling pipe 20 of a large diameter. A butterfly valve 18 for adjusting pressure is provided inside the assembling pipe 20. A vacuum pump 22 is provided at a discharge side of the assembling pipe 20, and a main exhaust pipe 24 of a relatively large diameter is connected to a discharge side of the vacuum pump 22. Atmospheric air and a gas within the processing chamber 2 are exhausted to the outside by this vacuum pump 22. A particle-measuring system 26 for counting the number of particles included in the exhaust gas is provided in the middle of the main exhaust pipe 24.
FIG. 19 is a diagram showing a cross-sectional configuration of the main exhaust pipe 24 provided with the particle-measuring system 26.
The particle-measuring system 26 has a laser beam irradiator 28 for emitting laser beams L and a stopper 32 for suctioning the emitted laser beams L disposed opposite to each other so that a line connecting between the two units pass through a center O of the main exhaust pipe 24. Further, a scattered light detector 30 for detecting scattered lights SL generated by a collision of the laser beams L against particles P in the middle of the irradiation of the laser beams L, is disposed facing the center 0 of the main exhaust pipe 24.
Based on this arrangement, for measuring the particles, the scattered light detector 30 detects the scattered lights SL that are generated when the laser beams L irradiated from the laser beam irradiator 28 have collided against the particles P that move within the main exhaust pipe 24. The particle-measuring system 26 counts the number of the particles included in the exhaust gas based on this detection.
According to the above-described conventional processing unit, the particle-measuring system 26 is provided on the main exhaust pipe 24 at the discharge side of the vacuum pump 22 that assembles the exhaust pipes 16 from the processing chamber 2 together. Of course, abnormalities of products adhere onto the inner walls of the exhaust pipes and blades of the pump and the valve due to the exhaust that occurs during the process from the processing chamber 2 to the particle-measuring system 26. These adhered abnormalities are peeled off irregularly, and these generate new particles.
As the particles generated irregularly are added to the discharged particles that have actually been generated from within the processing chamber 2, it has not been possible to accurately grasp the number of particles that have been generated from within the processing chamber 2.
Further, the exhaust gas is swirled within the exhaust pipe near the discharge side of the vacuum pump 22. Therefore, the same particles cross the laser beams repeatedly, and they are counted by a plurality of times.
In principle, the actual number of particles within the processing chamber 2 should be highly correlated with the count number based on the measurement of particle by the particle-measuring system 26. However, for the above reason, there is a very low correlation between the two data. Therefore, according to the conventional particle-measuring system, it has been difficult to accurately understand the state of particles actually generated from within the processing chamber 2.
Further, for example, when forming a thin film by a film-forming system, e.g., a CVD system, generation of particles which can be a factor of reduction in a yield of a product must be suppressed as low as possible. These particles are generally produced when an unnecessary film that has adhered to a surface of an internal structure, such as an inner wall surface of a process chamber, a mounting table or a shower head structure, flakes away. Therefore, after subjecting one lot (for example, 25) of wafers to film formation processing periodically or non-periodically, there is carried out etching processing which removes an unnecessary film by introducing a cleaning gas, such as ClF3, into the processing chamber, namely, cleaning processing. Generation of the particles in the processing chamber can be suppressed by this cleaning processing.
Since the cleaning gas is highly active, the inner wall surface of the chamber and other internal structures are also scraped away after the unnecessary film is removed, if cleaning processing is carried out longer than necessary. Therefore, it is very important to monitor the scraping state of the wafer during the processing, and determine an appropriate end point (point at which the etched film is removed), in order to terminate the cleaning processing with a just timing.
Description will now be given as to a conventional method for determining termination of the cleaning processing, i.e., the end point.
For example, there is a set a sequence to perform the cleaning processing for a predetermined time every time a predetermined number of, e.g., one lot (25) of wafers to be processed is subjected to film formation processing. At this moment, the predetermined number of wafers are actually subjected to the film formation processing, and an unnecessary film is deposited on the inner wall surface of the chamber or the internal structure. A cleaning processing time to remove the unnecessary film or an interval of execution of cleaning is experimentally obtained, and the cleaning processing is carried out based on such a time or interval. At this moment, the cleaning processing time may be determined by utilizing a plasma monitor. When the unnecessary film is, e.g., a silicon oxide film and the internal structure is, e.g., stainless, the color (wavelength) of light generated differs depending on the plasma. Therefore, the color of the plasma varies at a switching part in accordance with etching. The point in time at which the etched film (unnecessary film) has been completely removed, thus exposing a substrate (internal structure and the like) underneath, is referred to as “just etch”.
In actual cleaning processing, the cleaning processing is not terminated at just etch, and is continued for a predetermined period. In order to completely remove an unnecessary film which has adhered to a part where removal of the unnecessary film is difficult, as compared with a mounting table surface where removal of an unnecessary film is easiest, over etching, in which etching processing is prolonged for a predetermined period after the just etch point is carried out, and then the cleaning processing is terminated.
The over etching period is approximately ½ the time required from start of the cleaning processing to the just etch point. Therefore, if 300 seconds are required from start of the cleaning processing to the just etch point, cleaning processing continues for a further 150 seconds, thus cleaning processing reaches the end point after performing etching for a total of 450 seconds.
However, in the actual processing, there is rarely a case that one lot (for example, 25) of wafers to be processed is periodically supplied and manufactured. Therefore, when one lot slightly exceeds 25, several wafers are processed in the last processing. Furthermore, there may be a case that only a few wafers are subjected to film formation processing, and the film-forming system stays in the idling state for a long time until the next wafer to be processed is supplied, and the adherent unnecessary film may possibly change its nature in the processing chamber. Therefore, the cleaning processing is necessarily executed before entering the idling state.
In such a case, the thickness of the adherent unnecessary film is slightly less than the predetermined film thickness. Thus, by executing the regular cleaning processing mentioned above, the inner wall surface in the chamber or the internal structure, e.g., the surface of the mounting table, a shield ring, a shower head structure and others may be scraped away by excessive etching, or the surface of that member may be damaged by etching or corrosion. There occurs a problem that the duration of life of the internal structure is shortened by this damage. When a frequency of replacement of the internal structure becomes high, an operating rate of the system is deteriorated, and the throughput is lowered, which results in a problem that a product cost is adversely affected.